This invention relates to the measurement of the salinity of water, and particularly to the measurement of salinity using an electrical conductivity or resistance measuring system.
The electrical conductivity of sea water is related to the concentration of salt ions in the water and can be derived with considerable accuracy when the temperature of the water is known. Much effort has been spent establishing formulas that relate electrical conductivity, temperature, and ionic concentration. Since salt concentration, in the absence of mixing, is a conservative property away from the ocean's surface, it may be used to identify water masses in the oceans, and to track their motions. This is true both for large global scale motions that occur very slowly in time, and also for small scale motions that result in slight vertical mixing.
The investigation of large scale motions requires measurements of high accuracy if data from widely different times or places are to be compared. Investigation of small scale mixing processes requires high spacial resolution since mixing results from molecular diffusion over very small distances. For practical measurement of both types of motions it is valuable to have a measurement system which is capable of rapid measurement, is small, and can be battery operated in locations remote from an external power supply.
There are currently in use conductivity measuring systems which cause current to flow through the electrolyte either by induction or conduction. Inductive devices, while being highly stable, suffer from poor spacial resolution. On the other hand, conductive devices are capable of high spacial resolution but are often unstable due to changes in electrode polarization or electrode fouling. In these devices, electrode polarization effects are often reduced by making a four-electrode measurement and also by placing non-conducting barriers near the electrodes to increase the electrical current density thereby increasing the solution resistance relative to the electrode polarization. Electrode fouling effects are reduced by locating the voltage electrodes in areas of low current density.
Two patents relating to a salinity measuring system utilizing four electrodes are issued to Thomas M. Dauphinee, et al. in U.S. Pat. Nos. 3,963,979, issued on June 15, 1976, and 4,511,845, issued on Apr. 16, 1985. Both of these systems comprise laboratory based, relatively elaborate systems in which a glass tube containing specimen salt water is immersed in a temperature controlled bath. The four electrodes contact the solution in the specimen tube, and the salinity of the solution is measured by the application of a continuous square wave pulse train to the saline solution to create a current and then measuring the voltage drop caused by that current in a manner somewhat analogous to that of the instant application.
Another prior art device is disclosed in U.S. Pat. No. 3,939,408, issued Feb. 17, 1976, to Neil L. Brown. This patent is also somewhat analogous both the above-referenced systems and the instant system, and sets forth a T-shaped probe with a very particular arrangement of electrodes, for immersion and current production in a saline solution.
There is a need for a portable system for measuring salinity, which is small and lightweight, and which is designed to inhibit polarization and electrode fouling, uses a minimum amount of current, and produces a very accurate measurement.